Stabilising a Business-Critical Legacy ERP

Having completed a Strategic IT and Maturity Assessment for a group operating predominantly in the energy sector, TDS was retained to deliver the follow-on Stabilisation Phase.

Where the assessment diagnosed the problem, this engagement addressed it: turning the assessment’s roadmap into delivered change on the group’s business-critical, internally built ERP, stabilising and incrementally modernising the system while it remained in daily use.

The Challenge

Like many established organisations, the group had grown to depend on a core system built and maintained in-house over many years. As the business expanded, acquiring and onboarding new companies, that system came under constant change pressure, and years of accumulated architectural debt and manual delivery processes had become the single biggest barrier to keeping pace.

Two constraints shaped the work. First, the system is depended on daily, so modernisation could not disrupt users or pause essential bug-fixes, onboarding and new feature requests. Second, the group needed to raise the system toward the standard of modern commercial alternatives without the cost and risk of a full rewrite. The task was therefore to stabilise and refactor the highest-priority pain points in place, strengthen the delivery process around the system, and ground future investment in a clear, evidence-based understanding of user needs across a diverse set of businesses.

Solution and Implementation

Strategic Approach

TDS established a dedicated Stabilisation stream, ring-fenced from business-as-usual so that modernisation would not be crowded out by day-to-day demand. This directly enacted the assessment’s recommendation to separate improvement work from ongoing maintenance. The engagement has run as a sustained, multi-sprint programme across three coordinated workstreams.

Engineering and stabilisation.

TDS stood up a dedicated Stabilisation environment with an automated CI/CD pipeline, then refactored the system incrementally, targeting the highest-impact areas first rather than attempting everything at once. In place of a risky big-bang rewrite, the team applied a strangler fig approach, using CQRS (Command Query Responsibility Segregation, which separates read and write operations) to build modern, well-tested code paths alongside the existing system and migrate functionality across to them piece by piece. Work concentrated on the modules causing users the most friction and carrying the greatest architectural debt, replacing fragile legacy patterns with cleaner, better-tested implementations. Three principles governed the approach:

• No disruption to users: new and legacy code paths ran side by side, so existing processes kept working throughout the migration.
• Faster and safer delivery: automated testing and deployment reduced release risk.
• Incremental modernisation: functionality was moved across gradually, never replacing the whole system at once.

Business analysis and delivery uplift.

In parallel, TDS reconstructed the client’s legacy backlog items into structured user stories, giving developers and stakeholders clear, testable requirements for the first time. TDS also worked alongside the client’s own business analysts, running weekly BA sessions, delivering BPMN 2.0 process-mapping training, and introducing User Story Mapping, a technique the client team now uses independently to size and plan new development work.

User research and product-led insight.

Because the system serves a wide range of businesses across the group, TDS ran a dedicated user research stream to ground modernisation in real user needs rather than assumption. Using mixed methods (group workshops, one-to-one interviews and surveys), a TDS user researcher engaged users across the group’s diverse businesses, spanning finance, procurement, operations, engineering and people teams. The research built user personas, surfaced and prioritised common pain points and improvement opportunities, and captured how users actually experience the system. These findings feed directly into the epics and user stories driving the next phase of product-led delivery, and highlight improvements that deliver value across the group as a whole rather than for any single business in isolation.

Key Deliverables

• A dedicated Stabilisation environment and automated CI/CD pipeline
• Incrementally refactored high-priority modules and workflows
• A baselined, structured user-story backlog for the priority areas of the system
• Agile BA coaching, BPMN 2.0 training and User Story Mapping enablement for the client team
• A mixed-methods user research programme: personas, prioritised pain points and improvement opportunities feeding a product-led backlog

Results and Outcomes

• Priority technical debt addressed incrementally, with no disruption to the live system or its users.
• Legacy tickets transformed into clear, developer-ready requirements, improving delivery
• predictability.
• Modernisation grounded in evidence,: where user research across the group’s businesses turned scattered, anecdotal frustrations into prioritised, user-validated requirements, enabling a genuinely product-led approach.
• Measurable uplift in the client’s own delivery capability, with its business analysts now self-sufficient in story mapping and process mapping.
• A repeatable, low-risk delivery model that lets modernisation continue alongside acquisitions, fixes and feature requests, resolving the bottleneck that originally prompted the engagement.